Anak Krakatau's Alert Level Has Been Raised. Here's What That Means

This picture taken with a long exposure shows lighting bolts striking along the Sunda Strait between the Indonesian islands of Java and Sumatra in Rajabasa, in Lampung province, on December 25, 2018, three days after a tsunami - caused by activity at a volcano known as the 'child' of Krakatoa - hit the west coast of Indonesia's Java island. (MOHD RASFAN/AFP/Getty Images)

Getty

After the devastating, volcanically-generated tsunami in the Sunda Strait on December 22nd, I attempted to gather everything we knew about the state of Anak Krakatau to cautiously explain what may happen next. One of the chief concerns of Indonesia’s national disaster and management agency, the BNPB, is that another section of the 101-year-old volcano could collapse into the sea and trigger another tsunami.

Understandably, they’ve been keeping a close eye on Anak Krakatau in the past few days, and their latest assessment has been summarised in a new press release, which I’ll explain for you here. The key part is that the alert level has been raised due to the increased volcanism at the site, but that in no way suggests a tsunami is more – or even less – likely than beforehand.

What’s Anak Krakatau Doing Now?

The flank collapse that triggered the tsunami, which coincided with a large explosion and prominent ash column at the site, appears to have taken the pressure off the underlying magma reservoir somewhat. That means plenty of magma is now coming into contact with seawater and creating frequent, violent blasts – a form of molten fuel coolant interactions, or MFCIs.

MFCIs don’t always have to be violent, but when you release pressurised magma into shallow bodies of water, and you mix the two in just the right way, the cooling reaction is extremely rapid, and plenty of heat is released all at once. This release of energy is aggressive enough as it is, but the water plays a key role too. When magma comes into contact with water, that water is vaporized. This dramatically increases its volume and generates explosions, often flinging fresh volcanic material into the air.

The chaos unfolding at the site makes it difficult to properly quantify anything, but it appears that most of the volcanic activity at the fractured Anak Krakatau does involve magma-water interactions, with fresh lava and volcanic debris continually escaping to the surface world. That’s a style of volcanism known as phreatomagmatism.

In fact, a fair amount of it could be described as Surtseyan, a type of eruption named after Surtsey, an Icelandic volcano that quickly emerged from the sea towards the end of 1963. Far from just involving MFCI explosions, you also tend to a few other features too.

The eruption of Anak Krakatau continues. Strombolian activity. Other videos have shown evidence of magma-water interactions, looking Surtseyan in style at times. The risk of further collapses at the volcano remains. https://t.co/SPp3PMSp9q

These include “cypressoid” jets – elongated, curved ejecta tails of debris, propelled by explosions. These are also known as cock’s tail or rooster tail jets. At the bases of those skyward plumes of ash, you can also spot explosively-generated, expanding rings of hot ash and gas. These "base surges" track along the surface of the water much like the laterally moving, ground-based blasts you get when you detonate a nuclear weapon above ground.

As Boris Behncke, a volcanologist with Italy’s National Institute of Volcanology and Geophysics (INGV), suggested on social media, perhaps all this volcanic activity is Surtseyan. The cone and conduit that focused the magma upwards into the air has now been destroyed, and the primary vent for Anak Krakatau might be entirely submerged beneath the waves. That means that everything we see could be generated exclusively via underwater MFCIs.

Fajar Hidayatullah, 9, poses with his colour drawing at a relief centre in Kalianda in Lampung province on December 27, 2018, after he was evacuated from Sebesi island following the December 22 tsunami. (MOHD RASFAN/AFP/Getty Images)

Getty

What Has the Alert Level Changed?

In the past few days, the volcanic activity at the site has become more pronounced and explosive in general. Before, a danger zone – into which no-one is permitted – stretched two kilometres (1.2 miles) from Anak Krakatau. Now, as of the morning of December 27th, it’s five kilometres (3.1 miles).

No-one lives on the volcano, but – tsunami risk aside – it can be dangerous to those living on the nearby islands of Sumatra and Java. There, the ash can cause anything from mild irritation to eyes and lungs to serious conditions in those with cardiovascular or respiratory ailments. Getting a boat to see the eruption up close is far more hazardous, with explosive volcanism and perhaps even a lava bomb or two easily capable of cutting people’s lives short.

At the same time, the ash cloud can prove extremely dangerous to aeroplanes by clogging up and partially melting their engines. As a precautionary measure, all flights that would have gone through the area have been diverted until further notice.

The alert level has been bumped up from Level II to Level III, the second-highest. Alert levels can be deeply confusing for many, not least because different standards of volcanic danger signalling are used by different countries. Even within those countries, the alert levels’ meanings vary too.

In the US, for example, there are separate alert statuses for aviation and ground-based hazards. Designed to cover an extremely diverse range of volcanoes, from the lava-making effusive ones (i.e. Kīlauea) to the explosive ones (i.e. Mount St. Helens), the American volcanic alert system can sometimes create some confusion. A RED alert, for example, means that “major volcanic eruption is imminent, underway, or suspected with hazardous activity both on the ground and in the air.” What that literally means for a volcano like Kīlauea, however, is fairly different from what you’d expect to see at a volcano like Mount St. Helens.

With that in mind, when you see that a volcano’s alert level has increased, don’t panic. In general, it means that the volcanic activity typical for that particular volcano has amped up, and the relative risk to people and property has potentially risen somewhat.

That doesn’t mean that the BNPB thinks a near-future tsunami is now more likely at Anak Krakatau, but that still remains a distinct possibility. Just to be safe, the BNPB ask that communities stay away from the coasts for the time being, just in case another does take place. (Incidentally, if you’d like to know what it was like to live through the December 22nd tsunami, I’d suggest reading Norwegian photographer Øystein Andersen’s first-hand account of the catastrophe.)

This alert level increase is, in simple terms, a way of saying “hey, let's be careful, that volcano is now at a heightened state of activity.” Yes, the blast that was concurrent with the tsunami was pretty darn big, but the eruptions taking place between September and October of this year were, on average, larger. The alert level has been raised today because the Surtseyan activity since the 22nd has not only continued but has been increasingly more prolific.

This picture taken on December 26, 2018 shows the Anak (Child) Krakatoa volcano erupting, as seen from a ship on the Sunda Strait. (STR/AFP/Getty Images)

Getty

So, What’s Next?

Hard to say. Each volcano is idiosyncratic, so you can’t look at similar styles of volcanism around the world and use that to predict what will happen next at Anak Krakatau. This volcano is also extremely young, which means there isn’t too much data as to how it behaved in the past. As such, it’s a highly unpredictable entity, which is why the BNPB is ready for whatever may come.

While people are being evacuated from the affected areas, search-and-rescue efforts are still ongoing. A team on the ground managed to even rescue a stranded turtle that had been quickly washed onshore by the tsunami.

Sutopo Purwo Nugroho, the head of the BNPB’s communications outfit, can have the final word here. Asking people to use information from trusted, official sources only – as trustworthy journalists and science communicators do – he also advises people to “remain calm and increase their awareness.” As ever, don’t use unofficial or suspicious-looking sources of information, and don’t believe any rumours of any kind. Whatever you do, be careful not to spread misinformation.